Humidification / dehumidification
Most data-centers will require humidification or dehumidification at some point in its operational life the typical need will be for humidification during the colder winter conditions. As mentioned previously data-centers that are to be operated with controlled humidity conditions should be constructed as a virtually hermetically sealed environment. To accomplish this proper practice calls for a continuous vapor barrier and any penetration to be sealed to prevent moisture migration into or out of the data-center, i.e. 4 mil polyethylene sheeting in the walls, concrete sealed with a vapor barrier coating, the ceiling should have either poly sheeting or an applied vapor barrier paint or sealer, and all cable and electrical penetration voids filled. Unfortunately these practices are hardly ever employed and many data-centers are unable to maintain the desired humidity levels. The requirement to maintain humidity in data-centers is typically needed to reduce the potential of Electrostatic Discharge (ESD) which can damage solid state devices in data-center equipment. Historically humidity control was much more critical in days of open tape drives and punch cards as low humidity would cause breakdown of the iron oxide coating of magnetic data storage tapes and excessive dust from punch cards that would foul machines. Today the concern is for ESD and to keep printed circuit boards from excessive expansion and contraction. Current ASHRAE standards which are endorsed by most manufacturers allow rooms to run at a wider humidity range than previous standards, however the rate of change in humidity is of more concern and needs to be gradual in the new standards.
Dehumidification tends to be required much less, and in most applications is not essential, outside of preventing the humidity rate of change from exceeding ASHRAE and/or manufacturers standards the primary factor would be to insure that the conditions are non-condensing, which would rarely happen in a data-center.
Humidification systems use a variety of ways to produce water vapor, steam generating canister, infrared (IR), ultrasonic, high pressure mister, spray, process steam, and evaporative media.
Steam generating canister humidification systems are the probably the most common. Moisture is produced as high temperature, pure vapor steam by energizing electrodes immersed in water inside a specially constructed canister. Control electronics monitor operation and regulate steam output at design capacity and maximize electrode life. As a canister produces steam the surface of the electrodes become coated with the minerals from the water eventually rendering the canister useless and the canister needs to be replaced. The benefits of canister type humidifiers is they are fairly simple, produce pure water vapor, are easily integrated into systems in size and controls and are of reasonable cost. The cons of canister type humidifiers is the canister is a wear item and requires replacement after a certain number of hours in operation. Also canister humidifiers have high electrical consumption and produce hot steam which increases the load on the cooling equipment. Canister humidifiers operate at a fixed output and take a short period of time to reach full output.
Infrared (IR) humidifiers have many of the same operating characteristics as canister type humidifiers but produce steam by flashing the water into vapor with high intensity lamps positioned over a pan of water. Maintenance consists of periodically cleaning the water pan as needed and replacing lamps when they burn out. IR systems produce pure water vapor, and are at production capacity within seconds of being energized. IR systems produce heat which must be removed by the cooling equipment and have high electrical consumption. IR systems are fairly small but need to be installed in the air stream to be effective.
Ultrasonic humidifiers produce "fog" by injecting a high frequency waveform into a steam of water, the fog is readily absorbed into the air stream and converted to vapor. Ultrasonic humidifiers perform adiabatic cooling from the evaporation of the micro fine water droplets. The primary benefit of ultrasonic systems is the production of humidity with low energy consumption. The cons of ultrasonic systems are that the feed water needs to be very clean requiring high level filtration to prevent fouling of the ultrasonic element and to keep minerals that are dissolved in the water from being introduced to the air stream and making a fine dust load in the data-center.
High pressure mister systems are similar to ultrasonic systems except instead of a ultrasonic element they use a high pressure pump and special nozzles to produce a fine mist. Typical mister systems require a centralized pump system and piping to the point of use with automatic valves to deliver humidification. Like ultrasonic systems mister systems produce humidity for low energy consumption as the primary advantage. Another advantage is that mister systems have large capacities requiring only one pump to deliver significant amounts of moisture to the space. Disadvantages of mister systems are that they aren't well suited for small capacity systems, require clean water like ultrasonic systems and have a greater potential of producing water carryover in the case of nozzle fouling. Also they will produce fine dust from any dissolved minerals that are in the feed water.
Spray systems use low pressure nozzles to introduce droplets into the airstream, some of the water will evaporate adding the desired moisture and providing some cooling through the adiabatic process . Spray systems will typically use a recirculated water system with a section of duct that forms a chamber where the air has time to absorb the moisture. spray systems must have mist eliminators to remove droplets from the airstream at the end of the chamber. Properly configured spray systems produce pure water vapor with minimal energy consumption compared to steam generating type systems. Cons of spray systems are the need for the chamber area, high risk of carry over, concerns about biological growth in the recirculated water system, and the maintenance required in filtration and treatment of the water system.
Mister systems along with spray and wetted media are well suited for outside air economizer systems where large amounts of moisture need to be added to the outside air as it is brought into the data-center, however care must be taken in the design of such systems if they will be operating when ambient temperatures are below freezing. Outside air should be mixed with return air in such a way that the airstream is uniform in temperature, then the humidity injected into the airstream, this configuration will prevent the possibility of ice formation, or freeze damage to piping systems.
In outside air economizer systems the possibility of water carryover is increased because of the large amounts of moisture that must be added to the air stream, system design must address this issue to prevent water from entering the data-center and adversely affecting operations.
Process steam systems will utilize either a dedicated process steam generator, or process steam from a central plant. Process steam systems are not very common as few facility's have a central process steam plant and the dedicated system is considered less piratical than canister or IR. Pros of process steam are they provide pure water vapor, and can have very high capacities. With central systems the control is very simple and operational cost can be less if powered by fossil fuel fired boilers as opposed to electric powered systems, also some of the heat associated with producing process steam can be located outside the data-center reducing the heat load on the cooling equipment and associated energy consumption as opposed to integrated systems. Cons of process steam systems are, high energy use, the cost of piping systems to convey the steam into the data-center with the associated energy loss if the steam generating system is outside the room, water treatment required to keep the system operating efficiently, and potential operating permits in the case of boilers operating under pressure.
Evaporative media provides humidification by pumping recirculating water over a pad of woven fibers or specially treated corrugated paper (celdek) to keep the media wet while air flows through the media. The media absorbs water by capillary action and maximizes contact between the airstream and the wet medium. To ensure that all surfaces are wet, more water is usually pumped than can be evaporated and excess water drains from the bottom of the media into a sump. An automatic refill system replaces evaporated water. Pros of the evaporative media system are production of pure water vapor, low energy consumption, mechanically simple construction, adiabatic cooling, and relatively simple maintenance. Cons of evaporative media are restriction of airflow requiring more fan energy, and potential disruption of service if the media needs replacement or cleaning when installed in the primary air stream, a space in a horizontal section of duct to properly operate, and the potential for biological fouling.
Dehumidification can be accomplished by a few methods. Most CRAC and CRAH units incorporate a dehumidification mode of operation. First and most common is a compressorized direct expansion (DX) refrigeration system, and the second is the use of chilled water coils being operated at full capacity, and third is a desiccant wheel system with waste, or produced heat regeneration. In a typical dehumidification application there is a need for the system to reheat the air, however if the need for dehumidification from poor construction of the data-center allowing excessive infiltration of water vapor are not an issue, a properly configured dehumidification system in a data-center will have enough heat load that reheat systems are not needed. In reference to a system being "properly configured" a DX based dehumidification system should employ a high latent capacity system. Most dehumidification in data-centers is accomplished by operating a high sensible system at its maximum cooling capacity which drives the cooling coil temperature low enough to remove moisture from the air stream. Operating a system in dehumidification mode may over cool the space particularly if the base load on the system is less than the capacity of the system. CRAC units can encounter problems if required to dehumidify and the base load is less than about 65% of rated capacity. Under these conditions the controls will activate the second stage of cooling which needs to be operated along with the first stage of cooling to effect dehumidification, if first stage is not operating the system will only cool the space not dehumidify because the coil temperature is not low enough to condense moisture out of the air without both stages of cooling operating. If the dehumidification system is based on a chilled water system the chiller must supply chilled water that is cold enough to operate the coil below the dew point of the air in the data-center. Problems occur in systems where the chiller operates on a reset schedule based on ambient conditions for energy conservation, if the need for dehumidification arises and the chilled water is too high the system will only cool the space without dehumidifying it and the temperature will be driven too low. This can set up a scenario where the cooling operates in full capacity and the reheat is energized consuming high amounts of energy without performing the dehumidification the control system is trying to accomplish.
Desiccant wheel systems are not commonly applied to data-centers and have a few significant disadvantages. First is the function of regenerating the desiccant using heat, the residual heat in the wheel will be rejected into the data-center airstream and will add to the heat load of the data-center. Typical conditions when data-centers need dehumidification are during warm, to hot, high humidity ambient conditions, at these operating conditions the cooling system may be operating near is design capacity and the additional heat load would need to be accounted for in the engineering of the cooling system. Another disadvantage to desiccant wheel systems is the mechanical configuration, by nature the wheel will provide a "leaky" connection with the outside environment and the mechanics of the wheel introduce several potential points of failure. Desiccant wheel systems can be much more energy efficient at dehumidification than DX systems, but if the data-center is properly setup the need for dehumidification will not be enough to realize a net benefit over a DX based system.
The need to control the humidity in the data-center requires an examination of the factors that cause the levels of humidity to change. Data-centers house electronic equipment that neither produces or consumes humidity, thus the factors that cause change in humidity levels are from "external" factors. One misconception in the industry is that CRAC and/or CRAH units routinely dehumidify when operating, if the units are installed and operating according to manufacturers instructions and the conditions are being maintained at ASHRAE standards the rated heat removal will be 100% sensible. Other external factors are personnel in the center which would add humidity, and fresh air to satisfy the needs of any personnel working in the center will add additional humidification or dehumidification requirements. Attention to controlling outside air should be given the proper consideration as most data-centers will operate without any personnel in them much of the time. During unoccupied periods outside air should operate at the minimum required to keep the space at a slight positive pressure over adjoining spaces to avoid uncontrolled forced infiltration of air. To avoid excessive outside air from being brought into the data-center the outside air system should control the introduction of outside air based on first controlling space pressure and second on CO2 levels. Water vapor in the atmosphere is like water anywhere, it seeks it's own level, thus when two adjoining spaces are at different levels of absolute humidity they will seek to equalize. Common construction materials like sheet rock walls and concrete allow significant moisture migration unless properly treated with coatings specifically manufactured to prevent water vapor migration. In the case of data-centers when the environment outside is lower in terms of absolute humidity the vapor pressure forces water vapor out of the data-center to the surrounding environment, and if the level is higher outside the data-center the reverse is true. Understanding the nature of water vapor migration helps to understand why proper attention to constructing data-centers with vapor barriers and outside air control is important.
In considering the previous discussion it is clear that the requirements for humidification and dehumidification are primarily a function of the shell construction and the attention to detail in the implementation of the mechanical systems, and the anticipated occupancy levels of the data-center. Conventional data-center cooling systems will be equipped with a standard capacity humidifier as an option and have a certain amount of dehumidification capacity available. Unfortunately this has led many to believe that the mechanical system will maintain the desired conditions in the data-center and fail to give proper consideration to the need for vapor barriers and sealing penetration voids in construction. This prevalent attitude has two major consequences. First is in cooler climates humidification systems operate more than they should causing a tremendous waste of energy from producing humidity and the added cooling load. Second when ambient conditions reach their extremes the systems can't keep up with the demand and the data-center exceeds the design operating conditions. In warm humid conditions the excessive operation of systems in the dehumidification mode also causes a tremendous waste of energy and reduction of sensible cooling capacity since systems may need to be configured to operate at a lower sensible heat ratio. This wasted capacity will require that a data-center installs extra capacity to cover the heat load, the increased cap-ex, and op-ex will far exceed the initial cost of installing a proper vapor barrier package in both scenarios. When considering the construction of a data-center the prudent path would be to address the humidity control as an independent part of the cooling system. Since most data-centers are built in phases the initial phase may not have enough installed capacity to meet the requirements of the shell causing excursions beyond design conditions until the center is fully built out.
IT Aire has designed a dew-point control system that works in conjunction with the IT Aire system that addresses the need of the data-center in a highly efficient package. By utilizing a wetted media humidification section and a high latent heat ratio dehumidification coil in a bypass configuration integrated into or as a independent component an IT Aire system will provide the most economical humidity control.